The present invention relates to the x-ray tube art. It finds particular application in conjunction with x-ray tubes having straddle bearing anode assemblies for use with CT scanners and the like and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in conjunction with other bearing assemblies used in conventional x-ray diagnostic systems and other penetrating radiation systems for medical and non-medical examinations.
Typically, a high power x-ray tube includes an evacuated envelope made of metal or glass, which holds a cathode filament through with a heating current is passed. This current heats the filament sufficiently that a cloud of electrons is emitted, i.e., thermionic emission occurs. A high potential, on the order of 100-200 kV, is applied between the cathode and an anode assembly, which is also located within the evacuated envelope. This potential causes electrons to flow from the cathode to the anode assembly through the evacuated region within the interior of the evacuated envelope. The electron beam strikes the anode with sufficient energy that x-rays are generated. A portion of the x-rays generated pass through an x-ray window on the envelope to a beam limiting device or collimator, which is attached to an x-ray tube housing. The beam limiting device regulates the size and shape of the x-ray beam directed toward a patient or subject under examination, thereby allowing images of the patient or subject to be reconstructed.
In addition to generating x-rays, the impact of the electrons on the anode generates thermal energy. In order to distribute the thermal loading and reduce the anode temperature, a rotating anode assembly is often used. In this system, the electron beam is focused near a peripheral edge of the anode disk at a focal spot. As the anode rotates, a different portion of a circular path around the peripheral edge of the anode passes through the focal spot where x-rays are generated. The larger the diameter of the anode, the greater the cooling time before the electron beam strikes the same spot.
Typically, the anode is mounted on a shaft and rotated by a motor or drive. The anode, shaft, and other components rotated by the drive are part of a rotating assembly, which is supported by a bearing assembly. Bearing assemblies found in most x-ray tubes today utilize either a cantilevered bearing arrangement or a straddle bearing arrangement, in which the bearings are mounted to straddle the center of mass of the anode target. While the straddle bearing arrangement is effective for reducing mechanical stress and anode vibration on the bearings, it provides a challenge to cool the bearings and bearing assembly, which are located in the center zone and surrounded by the hot target.
The bulk temperature of the target during operation reaches approximately 1200xc2x0 C., while the rotor temperature can reach about 500xc2x0 C. Because of the high temperature difference between the target and rotor, heat conduction from the target to the rotor through the shaft can be substantial. In addition, the rotor and bearing assembly receives heat radiated from the target, leading to a rise in bearing temperature. As a key component of the rotor, the bearings must operate properly. A malfunction of the bearings leads to anode wobble, causing a distortion of the x-ray image, defocusing of the focal spot, and mechanical failure of the x-ray tube.
One prior method of bearing cooling involves a passive mode of heat transfer. More particularly, heat is transferred from the bearing balls or rollers through the bearing housing to be dissipated by the cooling fluid from the stem of the bearing housing. This method of heat dissipation has limited heat removal capacity due to a limited area between the stem and the cooling fluid and relative low activity of the fluid in the vicinity of the bearing housing. Another prior device includes a bearing housing having internal passages through which cooling oil is pumped. However, these passages are relatively narrow, making it difficult to provide adequate circulation and circulation velocity of the cooling oil. When the cooling oil dwells for too long a period of time in contact with hot surfaces, it overheats and carbonizes.
The present invention contemplates a new and improved x-ray tube assembly having a rotating anode assembly with an anode cold plate, which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, an x-ray tube assembly includes an x-ray tube housing, a cathode assembly, and a rotating anode assembly. An insert frame, which is supported within the x-ray tube housing, defines a substantially evacuated envelope in which the cathode and anode assemblies operate to produce x-rays. An anode cold plate, which is disposed between the anode assembly and one end of the x-ray tube housing, is in thermal communication with the anode assembly.
In accordance with a more limited aspect of the present invention, the anode cold plate includes a cover having a top surface in thermal contact with the anode assembly and a basin connected to a peripheral portion of a bottom surface of the cover. The anode cold plate also includes an inlet tube disposed at a first end of the basin, which receives dielectric liquid coolant, and an outlet disposed at a second end of the basin.
In accordance with a more limited aspect of the present invention, the x-ray tube assembly further includes a heat barrier substantially surrounding and spaced apart from a bearing housing.
In accordance with another aspect of the present invention, a rotating anode x-ray tube includes an anode disk connected to a shaft, a bearing housing in which a plurality of bearing rotatably support the shaft, and a drive for rotating the shaft and anode disk. A heat barrier is substantially surrounding and spaced apart from the bearing housing. An anode cold plate assembly is disposed below and in thermal contact with the bearing housing and the heat barrier. A cathode is disposed opposite to and displaced from the anode disk. Further, the rotating anode x-ray tube includes an evacuated envelope within which the cathode, anode disk, shaft, bearing housing and heat barrier are at least partially disposed.
In accordance with a more limited aspect of the present invention, the anode cold plate assembly includes a cover having a top surface on which the bearing housing and heat barrier are mounted and a basin mounted to a peripheral portion of a bottom surface of the cover to define a chamber therebetween. An inlet tube is disposed at a first end of the basin for receiving liquid coolant and an outlet is disposed at a second end of the basin through which the liquid coolant exits the basin.
In accordance with another aspect of the present invention, an x-ray tube assembly includes a housing, an insert frame supported within the housing which defines an evacuated envelope in which a cathode assembly and a rotating anode assembly operate to produce x-rays. The rotating anode assembly includes a bearing assembly within a bearing housing and a heat barrier substantially surrounding the bearing housing. A method for cooling the bearing assembly includes positioning an anode cold plate in thermal contact with both the bearing housing and the heat barrier. In addition, cooling fluid flows into contact with an extended surface of the anode cold plate.
One advantage of the present invention resides in improved cooling of the bearing assembly.
Another advantage of the present invention resides in a cold plate integrated with the bearing housing.
Another advantage of the present invention resides in a heat barrier integrated with the bearing housing.
Yet another advantage of the present invention resides in enhanced liquid coolant flow velocity.
Other benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments.